1. Field of the Invention
This invention relates to an image detecting apparatus. More specifically, the invention relates to an apparatus for detecting selected patterns, such as characters printed on an object.
2. Description of the Related Art
Generally in this type of apparatus, an imaging device, such as a CCD (charge coupled device) camera, picks up an image of an object and produces an image signal. In the case of detecting characters printed on the object or flaws which exist on the object, the image signal from the imaging means is converted into a binary signal.
After conversion into the binary signal, a high level of the binary signal corresponds to characters or flaws on the object. Accordingly, when the binary signal is compared with a reference signal, a selected output signal corresponding to characters or flaws can be detected. The selected output signal is supplied to a displaying circuit to display whether a specified type of image appears in the image of the object. In this apparatus, however, if the object is a metal plate and characters are printed on this metal plate, a subtle variable-density image is formed on the metal plate in accordance with the finishing state of the surface of the metal plate, the arrangement of the imaging device and the illumination of the plate. Consequently, it is difficult to extract a signal corresponding to characters or flaws because the signal level corresponding to characters or flaws is almost equal to that generated by the metal plate as a background.
When the image to be detected is printed matter other than characters, the same problem as mentioned above occurs if the brightness of the image to be detected is similar to that of the object. According to the manner of this related art, it is more difficult to detect a selected image on the object when the object is being moved, such as during a production process or assembly line in a factory.